Losing all your money isn't fun so even if you gamble for free and win real cash you should try to gamble effectively.
Whenever you gamble there are always odds of winning and losing. Most lotteries have a huge 50% advantage over the player but offer the highest jackpots. Blackjack and other table games have a smaller advantage over the player but offer smaller wins often times only doubling your money.
Poker is very fun and profitable way to gamble because you can bet more money on a good hand and fold hands that are typically not profitable. NoPayPoker offers poker that can be played for free and you can win real cash. Their cash games also have no house advantage which can mean the difference between winning and losing.
Odds are meant to work best over the long run. Suppose your odds of winning a coin toss are 50% and so are your odds of losing. If you only flip the coin 20 times you might win more than 50% of the time. This does not mean you have an advantage but you simply haven't flipped the coin enough times to see the accuracy of the odds. Flip the coin 1,000,000 times and you will see that you would win 50% of the time over the long run plus or minus 1%.